Recently, as one approach to the promotion of convenience of an automobile, a device called "car-to-car distance alarm device for an automobile" which, in observance of the concept of active safety directed to prevention of an accident, is adapted to measure the distance from the user's own automobile to a preceding automobile, judge the danger of collision of the two automobiles and, on detecting the danger, issue an alarm to urge the driver's attention has been developed and adopted for practical use.
In the conventional car-to-car distance alarm devices for use in an automobile, the judgement of the presence or absence of a dangerous situation is generally accomplished by emitting one beam or three beams of laser light thereby measuring the distance to an object for measurement and comparing the measured distance with the safety car-to-car distance in accordance with one formula of decision. Some of these devices, however, are adapted such that the user is allowed to adjust freely the timing of issuing an alarm by adequately varying the safety car-to-car distance used in the formula of decision (refer to JP-A-58-10,198, for example).
In the car-to-car distance alarm devices for an automobile which are constructed as described above, however, one formula of decision is relied on to judge the question whether or not the alarm is to be issued. Generally speaking, this fact is suspected to bring about various disadvantages when the user has set the issuance of an alarm difficult. When the user's automobile is involved in traffic congestion, for example, the user is apt to set difficult the issuance of an alarm with a view to avoiding an unwanted addition to the noise of alarm. This fact implies the possibility that the issuance of an alarm will fail notwithstanding the situation generally threatens high danger of rear-end collision. Even some of the devices of the type which do not allow the user to adjust the timing of issuing an alarm are adapted to refrain from issuing an alarm when the user's automobile is running at a speed not more than a fixed level for the purpose of avoiding frequent issuance of an alarm during a course of traffic congestion. They have the possibility of posing the same problem.
The conventional car-to-car distance alarm devices for an automobile emit the laser light in one beam or three beams for the measurement of the distance. They are, therefore, incapable of discriminating between such items as guardrails, road signs, and other facilities attendant on roads, slopes of mountains, and walls of buildings which obstruct the travel of the user's own automobile and such items as vehicles running ahead and obstacles on roads which can obstruct the travel of the user's own automobile and often tend to issue an erroneous alarm such as by a guardrail.
It is, therefore, an object of this invention to provide a highly reliable car-to-car distance alarm device for an automobile, which is capable of infallibly issuing an alarm in a state necessitating deceleration or manipulation of brakes, even in the case where the user has set the issuance of an alarm difficult.
It is a further object of this invention to provide a highly reliable car-to-car distance alarm device for an automobile, which can reduce incorrect alarms such as by a guardrail.